A living, microscopic, single-cell organism. Wild yeast spores are always floating in the air. Just when these wild spores first interacted with foods and liquids is uncertain, but we do know that Egyptians used yeast as a leavening agent over 5,000 years ago and that wine and other fermented beverages were made for millennia before that. It was in 1857 that France's famous microbiologist Louis Pasteur discovered that FERMENTATION was caused by yeasts. During fermentation, yeast converts food (in the form of sugar or starch) into ALCOHOL and CARBON DIOXIDE. In the production of wine, the conversion of yeast to alcohol is necessary for the final product, and carbon dioxide is what makes SPARKLING WINES effervescent. To multiply and grow, all yeast needs is the right environment-moisture, food, and a warm, nurturing temperature. Today, scientists have been able to isolate and identify the specific yeasts that are best for winemaking. Modern winemakers carefully choose the yeasts they use in combination with different varieties of grapes. Various yeasts have specific properties and are better suited for particular winemaking styles. For example, some yeasts produce less foam and are therefore well suited for BARREL FERMENTATION. Those styles of yeast that are resistant to cold temperatures are best for making white wines. Other yeasts ferment more rapidly, tolerate alcohol better, or impart flavors to the wine (some desirable, others not). Popular commercially available yeasts used today include Champagne, Epernay, Montrachet, Pasteur Champagne, and Steinberg. Rather than resorting to using cultivated yeasts, some winemakers prefer native yeast fermentation, which relies simply on natural wild yeast spores. See alsoBRETTANOMYCES.